Reconfigurable Intelligent Surfaces (RISs), mainly acting as passive yet tunable smart reflectors, have been recently introduced as a cost- and energy-efficient means to boost the performance of various wireless applications [1]}, [2]}, [3]}, by altering the signal propagation environment [4]}, [5]}. Among them belong the radio localization and sensing applications [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, especially in high-frequency systems, e.g., millimeter Wave (mmWave), where it has been identified that the deployment of RISs can create additional signal propagation paths, serving as extra degrees of freedom for the design of localization schemes. It has already been widely accepted [12]} that RISs can provide a virtual Line-of-Sight (LoS) path from the Base Station (BS) to Mobile Stations (MSs) when the direct LoS path is spatiotemporally blocked; this situation can appear frequently in practice both in outdoor and indoor network deployments [13]}, [14]}. Actually, even when the direct LoS path exists, an RIS can further enhance the localization performance by leveraging spatial diversity. In fact, an RIS can be regarded as an additional reference node (a.k.a. anchor) in addition to the BS. In certain envisioned applications, RISs can enable localization even when a BS or an access point is not part of the localization mechanism [10]}, [16]}.
